The present invention relates to a method of and an apparatus for suppressing the degradation of a semiconductor element due to irradiation to increase the useful life of an electronic circuit.
It has been well known that a semiconductor element loaded in an artificial satellite or the like is exposed to cosmic rays, and thus is degraded or made defective. Of parts making up the electronic circuit of an artificial satellite, a semiconductor device such as an integraded circuit is most affected by radiation. Of integrated circuits, a CMOS circuit is most suited to be used in space environment because of low power consumptions. Metal oxide semiconductor field effect transistors (MOSFET's) making up the CMOS circuit, however, are readily degraded by radiation.
When an artificial satellite such as a communication satellite has been launched, the maintenance thereof is usually impossible. Accordingly, it is necessary to make the degradation of a semiconductor element due to cosmic rays as small as possible, thereby increasing the life of the electronic circuit. Thus, the electronic circuit of the artificial satellite is surrounded by an aluminum shield which also serves as a heat radiator, to make the total dose given to the electronic circuit as small as possible. The weight of the artificial satellite, however, is limited. Hence, it is impossible to make the thickness of the aluminum shield large without limitation. Accordingly, irradiation experiments on constituent parts of the electronic circuit are done on earth, and an allowable dose rate is determined from the results of the above experiments and the life of the artificial satellite. Then, the thickness of the shield is determined on the basis of the allowable dose rate.
When a device becomes faulty in space environment, it is not easy to repair the device. Accordingly, it is very important that the artificial satellite is provided with means for making the life of the electronic circuit which is surrounded by the shield, as long as possible.
In order to recover from the degradation of characteristics of the FET due to radiation, the following method has been proposed.
The degradation of characteristics of the FET due to irradiation is based upon a change in threshold voltage of the FET (hereinafter referred to as "threshold-voltage shift") and a reduction in channel mobility. Specifically, the threshold voltage shift due to irradiation is large and is considered to be a major cause for the erroneous operation of the FET. It is known that the threshold voltage of the FET can recover (or anneal) at an appropriate temperature. This fact can be used to recover from the degradation of the FET due to radiation. In more detail, the FET is put in high-temperature environment, that is, the FET is kept at a temperature of about 100.degree. C., to recover from the threshold-voltage shift.
The degradation of the FET depending upon a total dose has hitherto been estimated on the basis of irradiation experiments on earth, and the above experiments use a dose rate of 10.sup.2 to 10.sup.4 Gy/h, which is far higher than a dose rate of cosmic rays actually incident on the FET (that is, about 1 Gy/h or less). The FET is degraded more rapidly at a high dose rate than at a low dose rate. Accordingly, the results of experiments on earth have been considered to be reliable.
Recently, it has been known that the damage to and the recovery (that is, annealing) of the FET take place at the same time in radiation environment, and thus the degration of the FET depends upon a dose rate even when the FET is irradiated to a predetermined total dose. In more detail, the threshold-voltage of the FET is shifted in a negative direction at high dose rates and in a positive direction at low dose rates (refer to an article entitled "Total-Dose Radiation and Annealing Studies" by P. S. Winokur et. al., IEEE Trans. on Nuclear Science, Vol. NS-33, No. 6, 1986, pages 1343 to 1351). The reason for the above is considered as follows. The threshold-voltage shift is based upon 1 the negative threshold-voltage shift due to accumulation of trapped positive charges in the gate oxide film of the FET, 2 the recovery of the threshold voltage due to the recombination (that is, neutralization) of the trapped position charges with electrons, and 3 the positive threshold-voltage shift due to an increase in the number of trap levels formed in the interface between a silicon layer and a silicon oxide film. The negative threshold-voltage shift due to oxide-trapped charges and the positive threshold-voltage shift due to interface traps are both approximately proportional to a total dose. While, the recovery of the threshold voltage due to recombination is approximately proportional to the logarithm of time. Further, it is known that the speed of the recovery of the threshold voltage due to recombination increases as temperature is higher.
In a case where the dose rate is low, that is, the FET is used in space environment, the negative threshold-voltage shift due to oxide-trapped charges is naturally lessened by the annealing phenomenon due to recombination, and thus the characteristics of the FET are degraded mainly by the positive threshold-voltage shift due to interface traps. Accordingly, a conventional method of annealing the FET at a temperature of about 100.degree. C. to recover the threshold voltage, is not effective for the above case.
It is known that the interface traps can be eliminated at a temperature of 200.degree. C. or more (refer to an article entitled "Total Dose Hardness Assurance for Microcircuits for Space Environment" by P. Buchman, IEEE Trans. on Nuclear Science, Vol. NS-33, No. 6, 1986, pages 1352 to 1358). The allowable temperature of ordinary electronic parts is 125.degree.. When the electronic parts are annealed at a temperature of 200.degree. C. or more, the reliability of the parts is degraded. That is, such high-temperature annealing is unpractical.
Further, in order to anneal an electronic circuit at a high temperature, it is necessary to stop the operation of the electronic circuit, and moreover those parts included in the electronic circuit which are excellent in radiation resistance, for example, resistors and capacitors are kept at the high temperature. Thus, there is a fear of degrading the reliability of these parts.
As mentioned above, in the prior art, a major cause for the degradation of a semiconductor element which is used in space environment and is irradiated at a low dose rate, and the reliability of parts of the electronic circuit other than the semiconductor element are not taken into consideration. Accordingly, it is impossible for the semiconductor element to recover from degradation, and there is a fear of degrading the reliability of parts of the electronic circuit other than the semiconductor element.